Thinning followed by prescribed burning enhances growth and reduces vulnerability to drought of Pinus nigra spp. salzmannii

Increasingly frequent severe drought events are pushing Mediterranean forests to unprecedented responses. Lack of management leads to dense forests that are highly susceptible to water stress, potentially resulting in extensive dieback and increased vulnerability to other disturbances such as insect outbreaks and wildfires. Forest treatments such as thinning and prescribed fire can enhance tree growth and reduce tree vulnerability to severe disturbances. Here, we used tree rings to study growth and the physiological response of dominant black pine (Pinus nigra) forests to drought events in north-eastern Spain under different treatment combinations, including two thinning intensities (light and heavy, with 10% and 40% basal area reduction, respectively), two understory treatments (mechanical clearing and mechanical clearing plus prescribed burn of the debris), and an untreated control. Specifically, we studied basal area increment (BAI), resilience indices, and intrinsic water use efficiency (iWUE) using carbon and oxygen stable isotopes (δ13C and δ18O) before and after treatments. Our results showed that BAI and resistance to drought increased in the heavy-thin (burned and unburned) and light-thin burned units. Resilience increased in burned treatments regardless of the thinning intensity, while recovery was not affected by treatment. Within each thinning treatment, prescribed fire additionally increased BAI in the light-thin and resistance and resilience in the heavy-thin. The stable isotope analysis suggested that δ13C and iWUE were more influenced by year than treatment and that δ18O was positively correlated to growth during the drought after treatments. No change in iWUE among treatments was presumably linked to a proportional increase in both photosynthetic rate and stomatal conductance. These physiological processes were particularly enhanced in the heavy-thin (burned and unburned) and light-thin burned units, indicating that these trees were the least affected by drought after treatments. This study shows that management approaches primarily aimed at reducing wildfire hazard can also contribute to enhancing physiological activity of dominant trees under drought stress. By reducing competition both from the overstory and the understory, thinning and debris burning promote tree growth and vigour, and increase its resistance and resilience to drought. Active forest management should be encouraged to reduce vulnerability of sub-Mediterranean pine forests to severe and recurrent drought events in the face of climate change.